Exhaust after-treatment devices, such as an exhaust catalyst coupled to an exhaust of an internal combustion engine, reduce combustion by-products such as carbon monoxide, hydrocarbons, and oxides of nitrogen. For optimal performance efficiency, the temperature of the catalyst needs to be maintained at a temperature higher than the light-off temperature of the catalyst. Towards that end, engines may be configured with an exhaust heat recovery system for recovering heat from exhaust gas. The exhaust heat may be utilized for heating the exhaust catalyst during cold engine conditions, as well as for heating engine coolant which transfers heat to engine components and a vehicle cabin, thereby improving engine efficiency. Engines may be further configured with exhaust gas recirculation capabilities wherein exhaust gas is recirculated to the intake manifold to reduce exhaust NOx emissions and improve fuel economy. An exhaust gas recirculation (EGR) cooler may be coupled to the EGR system to bring down the temperature of the recirculated exhaust gas before it is delivered to the intake manifold.
Various approaches are provided for coordinating the supplying of heat to the exhaust catalysts, exhaust heat recovery, and EGR cooling. In one example, as shown in U.S. Pat. No. 8,240,294 Surnilla et al. discloses an engine system with a high-pressure EGR (HP-EGR) system supplying exhaust from the exhaust manifold, upstream of an exhaust turbine and exhaust catalyst, to the intake manifold downstream of an intake compressor. A HP-EGR cooler may be coupled to the HP-EGR system to cool the exhaust before delivery to the engine intake manifold. During cold-start conditions, a portion of exhaust may be recirculated via the HP-EGR cooler and an engine coolant may be circulated through the cooler to recover the exhaust heat and warm engine coolant. The heat recovered by the engine coolant may be used for heating engine system components. In order to increase the amount of heat recovery, the amount of HP-EGR recirculated to the intake manifold may be increased. Correspondingly, engine operating parameters may be adjusted in order to maintain combustion stability with the increase in the amount of HP-EGR.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, by harvesting exhaust heat upstream of the catalyst there may be an increase in engine cold-start emissions due to the HP-EGR cooler acting as a heat sink. In particular, during the engine cold-start, engine exhaust heat may be removed at the HP-EGR cooler, lowering the amount of exhaust heat that is received at a downstream exhaust catalyst. As such, this may delay catalyst light-off. Also, as exhaust is routed via a pre-catalyst cooler, there may be undesired changes in a target air-fuel-ratio perturbation reaching the exhaust catalyst. In alternate engine configurations, exhaust heat may be recovered downstream of the exhaust catalyst such that the exhaust heat may be first used for catalyst heating. However, in such an approach, the catalyst may function as a heat sink, and the heat recovered from the exhaust downstream of the catalyst may not be sufficient for engine coolant heating. Also, there may be a loss in the recovered heat due to the extended coolant lines. Further, if EGR is provided by recirculating exhaust from downstream of the catalysts, the reduced backpressure in the exhaust system, downstream of the catalyst, may adversely affect EGR flow to the intake manifold. While two separate heat exchangers may be used, one for exhaust heat recovery downstream of the catalyst, and one for EGR cooling upstream of the catalyst, the presence of multiple heat exchangers adds cost and complexity.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example engine method comprises: flowing a first portion of exhaust into an upstream exhaust catalyst via a heat exchanger in a bypass passage, flowing a second, remaining portion of exhaust into the upstream exhaust catalyst via a main exhaust passage arranged parallel to the bypass passage, and adjusting fueling on a per-cylinder basis as a function of the first portion relative to the second portion to provide a target exhaust air-fuel-ratio at the upstream catalyst. In this way, by positioning a combined exhaust heat exchanger and EGR cooler in an exhaust bypass upstream of the catalyst, the heat exchanger may be bypassed during catalyst heating, and after catalyst light-off, the heat exchanger may be used for exhaust heat recovery and/or EGR cooling.
In one example, an engine system may be configured with a single heat exchanger positioned upstream of one or more exhaust catalysts in an exhaust bypass passage disposed parallel to a main exhaust passage. A diverter valve coupled to a junction of the bypass passage and the main exhaust passage may be used to enable exhaust to be diverted into the bypass passage, and through the heat exchanger or diverted through the main passage to the tailpipe. An EGR passage may be coupled to the bypass passage downstream of the heat exchanger, and an EGR valve may be coupled to the EGR passage to control exhaust flow into the intake manifold. The position of the diverter valve and the EGR valve may be adjusted based on one or more of a catalyst heating request, exhaust heat recovery request, and EGR request. For example, during cold-start conditions, a position of the diverter valve may be adjusted so that exhaust may be directly routed to the exhaust catalysts, bypassing the heat exchanger. After catalyst light-off (activation) is achieved, the position of the diverter valve may be adjusted based on engine heating demands relative to catalyst heating demands so that a first portion of the exhaust may be routed to the catalyst(s) via the heat exchanger housed in the exhaust bypass, while a second (remaining) portion of exhaust may be directly routed to the catalyst(s), bypassing the heat exchanger. During exhaust flow through the heat exchanger, exhaust heat may be transferred to an engine coolant circulating through the heat exchanger, and the hot coolant may then be used for functions such as engine heating and cabin heating. Since the part of the exhaust entering the catalyst flows through a longer route, there may be unintended changes to the air-fuel ratio of the exhaust mixture reaching the catalyst. In particular, a desired exhaust air-fuel-ratio perturbation may be required at the catalyst to maintain catalyst functionality. In order to provide the requested air-fuel-ratio perturbation, combustion air-fuel-ratios may be adjusted on a cylinder-to-cylinder basis based on inputs from oxygen sensors coupled to the main exhaust passage, upstream and downstream of the heat exchanger. This allows, for example, a richer mixture, provided in the first portion of exhaust flowing to the catalyst via the heat exchanger, to be mixed with a leaner mixture, provided in the second portion of exhaust flowing to the catalyst bypassing the heat exchanger, thereby generating the desired exhaust air-fuel-ratio perturbation immediately upstream of the catalyst. When cooled EGR is requested, exhaust may be routed to the intake manifold via the heat exchanger and the EGR valve with the heat exchanger now operating as an EGR cooler. In alternate examples, exhaust may be drawn into the common heat exchanger from a location in between the exhaust catalyst. Exhaust flow through the heat exchanger may be regulated to maintain the temperature of the catalysts above their respective activation temperatures while recovering exhaust heat and/or providing EGR.
In this way, by providing the functions of an EGR cooler and an exhaust gas heat exchanger via a single heat exchanger, cost and component reduction benefits are achieved without limiting the functionality or capability of either system. By positioning the heat exchanger in a pre-catalyst or mid-catalyst location, EGR and engine coolant passage lengths may be shorter allowing for corresponding reductions in EGR transport delays and coolant heat losses. The technical effect of routing exhaust through the catalysts while bypassing the heat exchanger during cold-start conditions is that catalyst light-off will be unaffected, and cold-start emissions quality may be maintained. By routing the exhaust through the heat exchanger located before the catalyst and after catalyst light-off, exhaust heat recovery is increased, allowing for engine and cabin heating. For hybrid vehicles, exhaust heat recovery and expedited engine heating may allow the engine to be shut down within a shorter duration of time. The technical effect of adjusting the cylinder-to-cylinder air-fuel-ratio based on inputs from oxygen sensors coupled to the exhaust passage, upstream and downstream of the heat exchanger, is that phase shifting of exhaust air-fuel ratios may be provided at the catalysts even during heat recovery, thereby improving catalyst functionality. Overall, by expediting catalyst light-off, recovering exhaust heat, and providing cooled EGR using fewer components, emissions quality, engine fuel economy and performance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.